1. Field of the Invention
The present invention relates generally to the field of dynamic pressure generators in biomedical applications. More particularly, it concerns an apparatus and method for dynamically generating pressure waveforms representative of fluid pressure waveforms found in living bodies.
2. Description of Related Art
Pressure sensor modules and catheters have been used to measure the pressure of various bodily fluids, such as blood and intracranial fluid. Prior art systems have been developed to calibrate pressure sensors, particularly catheter tip pressure sensors, as a way to improve pressure readings of bodily fluids. These prior art systems fail, however, to provide accurate measurements. For example, prior art systems, such as U.S. Pat. No. 6,056,697, disclose methods and systems for calibrating catheter tip pressure transducers employed for measuring blood pressure within a living body. In U.S. Pat. No. 6,056,697, Owens et al. teach inserting the catheter tip pressure transducers in saline solutions at specified depths for the purpose of calibrating the device based on hydrostatic pressures. While Owens et al. describe calibrating systems using static pressure, they fail to teach the use of calibrating pressure-sensing catheters using dynamic pressures.
Another aspect of the shortcomings of this type of existing device for producing pressures is the lack of feedback relating to the preparation of the device for insertion into a body. For example, in fluid-filled catheters, it is desirable to remove all air bubbles from the system, as the presence of compressible air in incompressible fluid produces flattened waveforms when viewed on a monitor. Under static pressure conditions, the presence of air may be unnoticeable. Under dynamic pressure conditions, however, the compression and expansion of air bubbles in a fluid-filled catheter are noticeable. In order to eliminate these errors, researchers and other individuals that use fluid-filled catheters must practice preparing the catheters, including removing the air from the catheter. Some prior art systems, however, generally use static pressures, and therefore do not provide quality feedback as to whether the air was successfully removed from the system. It is therefore desirable to provide a system for providing immediate feedback relating to the preparation of a pressure-sensing device for insertion into a body.
Related to the existence of air bubbles in the system are the characteristics and performance of the pressure-sensing device. For example, in fluid-filled catheters, there is a certain volume of fluid inside the catheter. Associated with this volume of fluid is a certain frequency (dynamic) response of the system. At the natural frequency of the system, the pressure seems larger than normal, and above a certain frequency the fluid loses the ability to move fast enough to detect changes in the pressure waveform. The natural frequency for a fluid-filled pressure-sensing catheter for use in a mouse heart is much different than the natural frequency for a fluid-filled pressure-sensing catheter for use in a human heart. As the heart rate nears the natural frequency for the catheter, errors are introduced into the pressure measurements. Many prior art systems fail to address this type of error. Instead, these systems use static pressures to determine characteristics of pressure-sensing systems. It is therefore desirable to provide a system for determining the dynamic pressure-sensing characteristics and performance of pressure-sensing devices. It is further desirable to provide a system for determining the dynamic response characteristics of catheters, interconnecting tubing and fittings. It is further desirable to be able to measure dynamic fluid pressures without interference from the natural frequency of the pressure generator.
It is further desirable to be able to compare characteristics of different sensors. There are numerous drawbacks to using only static pressures for calibrating sensors and sensing devices. One drawback is that animal fluid pressures (e.g. blood pressure) are not generally static. Therefore, a static comparison of accuracy or consistency of sensing performance between two pressure sensors may or may not be correct. It is therefore desirable to provide a system for comparing the dynamic pressure-sensing characteristics and performance of two or more pressure-sensing devices.
It is therefore desirable to provide a system and method for verifying the accuracy of a pressure sensor for use in a living body.
U.S. Pat. No. 4,372,148, issued to Cieutat, discloses a pressure generator for producing a variable pressure, variable frequency waveform to test a blood pressure measuring system for resonance or damping. However, Cieutat uses a jet of gas on the paddle wheels of a turbine to turn a wheel to produce the desired pressure waveform. It is desirable to provide a dynamic pressure waveform generator that may generate a waveform directly without complex operating mechanisms.
U.S. Pat. No. 4,189,936, issued to Ellis, discloses a pressure generator for producing a pressure variation pattern, and further describes the use of a pressure generator in a high compliance pressure-simulating device. However, Ellis' use of a piston and a second chamber are counterintuitive to a high frequency (high compliance) dynamic pressure waveform, generator. It is desirable to provide a high frequency response dynamic waveform generator.